FIGS. 1 and 2 are views for explaining a prior art encoder disclosed in Japanese patent application publication (TOKKAIHEI) No. 08-261793. FIG. 1 is a perspective view of the prior art encoder and FIG. 2 is a plan view of the prior art encoder. In the figures, reference numeral 1 denotes a semiconductor laser shaped like a letter V, reference numeral 2 denotes two photodiodes, reference numeral 3 denotes a three-layer waveguide in which a core layer 32 is sandwiched by two cladding layers 33, the core layer 32 having a slightly higher refractive index than the two cladding layers 33, reference numeral 5 denotes a light beam emitted out of the semiconductor laser 1, reference numeral 80 denotes a total reflection mirror formed on a vertical surface of the three-layer waveguide 3, reference numeral 83 denotes convex edge faces formed on the waveguide, reference numeral 36 denotes a step formed on one convex edge face 83, reference numeral 51 denotes a parallel light beam emerging from the other convex edge face 83, and reference numeral 6 denotes a scale placed separately from a main body of the encoder.
Next, a description will be made as to the operation of the prior art encoder.
A light beam 5 emitted out of the semiconductor laser 1 is incident upon the core layer 32 of the three-layer waveguide 3 and is then collimated by one convex edge face 83 having a lenticular shape represented by a second-order curve after reflected by the total reflection mirror 80 on the way to the convex edge face 83. The light beam 5 is emitted as a parallel light beam 51 shown in FIG. 2. This parallel light beam 51 is made to be incident upon the scale 6 of grating type which is placed separately from the main body of the encoder. Two parallel light beams 51 are emitted out of the three-layer waveguide 3 and overlap each other on the scale 6 because the two convex edge faces 83 are formed symmetrically with the exception that one of them includes the step 36, and the first-order diffracted light beams diffracted by the scale 6 are incident upon the photodiodes 2.
By disposing the step 36 on the other convex edge face of the three-layer waveguide 3, a parallel light beam 51 emitted from the other convex edge face with the step 36 has two light beams 90 degrees out of phase with each other and is diffracted by the scale 6. Each of them has a +first-order or −first-order diffracted light beam interfere with the −first-order or +first-order diffracted light beam diffracted by the scale 6 that is emitted from the convex edge face 83 which does not have the step 36. They are incident upon to the different photodiodes 2, respectively. When the scale 6 of grating type moves in a direction of its pitches formed therein only by one pitch, the displacement of the scale 6 can be measured by observing a change in one photodiode 2 because a signal caused by the above-mentioned interference of a +first-order diffracted light beam and a −first-order diffracted light beam changes in 2 cycles. Furthermore, not only the relative displacement between the scale 6 and the main body of the encoder but also the direction is detected because the two sinusoidal signals acquired by the two photodiodes are 90 degrees out of phase with each other.
A problem with the prior art encoder constructed as mentioned above is that since when producing the three-layer waveguide 3 having a function of converging in the parallel direction to a surface of a substrate, the core layer 32 and the two cladding layers 33 are laminated alternately, it is impossible to form the three-layer waveguide 3 in one process and the manufacturing cost therefore rises. Another problem is that since the core layer 32 usually has a thin thickness ranging from several micrometers to tens of micrometers and does not have a function of converging an incident light beam in the perpendicular direction to a surface of a substrate, a reflected light beam from the object to be measured cannot be efficiently introduced into the three-layer waveguide 3.
The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a small-size and low-cost sensor head which does not need any three-layer waveguide with a high manufacturing cost.